This invention relates generally to control circuits and in particular to a ramp and pedestal phase control circuit.
In the past, various phase control circuits were known and widely used for controlling a supply of electrical energy to a load. One such past phase control circuit is of a half wave variety, as illustrated by the Dosch et al. U.S. Pat. No. 3,495,154, wherein a thyristor is gated part way through each positive half cycle of the source to supply portions of each successive positive half cycle to the load. With such half wave phase control circuitry, a substantial direct current component is delivered to the load. Another past phase control circuit is of the so-called Diac-Triac scheme, as illustrated in U.S. Pat. Nos. 3,403,315; 3,421,027; and 3,594,591. Such past Diac-Triac type circuits are also shown in the General Electric SCR Manual, Fourth Edition, pages 187-191. Briefly, in such Diac-Triac type circuits, a Triac or other type of controllable alternating current switching device was gated or enabled into its conducting state to effect the supply energy to a load during positive and negative half cycles of an alternating current source, and such gating of the Triac was effected by a Diac or other bilateral threshold device which became conductive when the voltage across a capacitor associated therewith exceeded a predetermined value regardless of polarity. This capacitor was charged during each half cycle through a variable resistance, and variations in such resistance determine the charging rate of such capacitor and, therefore, the particular time during each half cycle at which the Diac enabled the Triac. It is believed that one of the disadvantageous or undesirable features of these past Diac-Triac type phase control circuits was that they suffered from gain or sensitivity limitations. For example, when the aforementioned variable resistor which determined the time constant for capacitor charging was constituted by a photoconductor or a thermistor, inadequate amounts of phase control may have been experienced. A thermistor, for example, may typically have 3 to 4 per cent resistance change per degree centigrade, and such a change frequently was not sufficient to change the phase angle for firing the Triac enough to compensate for the temperature change experienced.
Still another of the past schemes for improving the gain or sensitivity of a phase control circuit is the so-called ramp and pedestal control circuit, as described in the aforementioned General Electric SCR Manual pages 191-196. For instance, the past known ramp and pedestal phase control circuits typically employed a relaxation oscillator type circuit which was energized by a bridge or other rectifying scheme to appropriately enable a unijunction transistor, and conduction through such unijunction transistor, in turn, was employed to gate a solid state switching device. Typically, a full wave rectified sine wave is clipped by a zener diode, and this clipped series of somewhat rectangular pulses was applied to the unijunction transistor and also was employed for charging a capacitor associated therewith. Of course, the voltage of the capacitor was employed to enable the unijunction transistor. The capacitor voltage was initially rapidly increased to a pedestal level, and thereafter such capacitor voltage was more slowly increased along a ramp until conduction by the unijunction transistor occurred. A resistive control element, e.g., a thermistor, varied the pedestal height so as to provide the sensitivity or gain required. The past known ramp and pedestal control circuits provided the desired sensitivity, but it is believed that at least one disadvantageous or undesirable feature thereof is that they were relatively complex and costly.